Multi-modality imaging systems are capable of scanning using different modalities, including but not limited to Positron Emission Tomography (PET) and Computed Tomography (CT). While some of the same hardware is utilized to perform different scans (e.g., an image produced by PET is processed and displayed respectively by the same computer and display as an image produced by CT), the data acquisition systems (hereinafter sometimes referred to as a “modality unit”) are different. For example, on a CT/PET system, a radiation source and a radiation detector are used in tandem to acquire CT data, while a radiopharmaceutical is typically employed in tandem with a PET camera to acquire PET data.
In such multi-modality systems, such as an integrated PET/CT system, there is an inherent registration of the PET and CT images the system acquires. Since the patient lies still on the same table during the PET and CT portions of the acquisition, the patient will be in a consistent position and orientation during the two acquisitions, greatly simplifying the process of correlating and fusing the CT and PET images. This allows the CT image to be used to provide attenuation correction information for the reconstruction of the PET image, and allows an image reader to easily correlate the anatomic information presented in the CT image and the functional information presented in the PET image.
This inherent registration assumes a perfect alignment of the PET and CT detector coordinate systems, or at least a known spatial transformation between the two coordinate systems. Proper PET and CT image registration also requires an alignment of the axial axis of the PET and CT coordinate systems not only with each other but also with the travel axis of the table that transports the patient during the PET and CT acquisitions. Even if the CT and PET are perfectly aligned, a misaligned table will produce different artifacts in the CT and PET volumes, which cannot be properly registered.
Prior multi-modality imaging systems utilize a dedicated volumetric quality control (VQC) phantom for image alignment. These VQC phantoms typically include a number of radioactive spheres or marbles embedded within the phantom which define a number of points. The VQC phantom is scanned once using both modalities, and any misalignment between the imaging modalities may be identified in the resulting reconstructed images of the VQC phantom. In particular, the positions of the points corresponding to the radioactive spheres in the VQC phantom which appear in the PET image and the CT image, for example, are used to register the images and/or align the PET gantry with the CT gantry. However, VQC phantoms are generally large, expensive, and challenging to maintain and store, which is unnecessarily burdensome considering that VQC phantoms are only occasionally used. Accordingly, it is desirable to provide systems and methods for aligning components of a multi-modality imaging system which does not rely on a dedicated VQC phantom.